1. Field of the Invention
The present invention relates to a piezoelectric transducer and the process for preparation thereof. More particularly, the present invention relates to a piezoelectric transducer usable preferably as an ultrasonic probe for diagnostic inspection which is conducted at a frequency band higher than 10 MHz and provides a clear image of a shallow portion from the surface of an object to be inspected, and the process for preparation thereof.
2. Description of the Related Art
The piezoelectric transducer is widely used as an ultrasonic probe for the intact diagnostic inspection of the abdomen and the chest of the human body. Such an intact diagnostic inspection is conducted by applying the ultransonic probe directly to the skin surface of the portion to be inspected to thereby enable a dynamic observation of the soft tissue structure of the portion.
Recently, there has been an increasing demand for an ultrasonic probe which can be swallowed into the stomach in order to obtain a clear image of the mucous membrane of the stomach wall. For this sake, the ultrasonic probe should present a high central frequency, usually higher than 7.5 MHz.
Generally the ultrasonic wave presents a larger propagation loss with higher frequency. Thus, the ultrasonic probe of a high frequency is not suitable for the diagnostic inspection of a deep portion of the human body. In the ultransonic probe, however, the resolution power is enhanced with a higher frequency so that the high-frequency ultrasonic probe can be preferably used for the dynamic inspection of a shallow portion such as the stomach wall. Accordingly, there have been developed ultrasonic probes having a higher frequency for obtaining correct and minute diagnostic information.
FIG. 1 is a perspective view showing a piezoelectric transducer of the prior art used as an ultrasonic probe for diagnostic inspection.
The ultrasonic probe shown in FIG. 1 includes a piezoelectric ceramic element 10 for electro-mechanical enery transduction. The piezoelectric ceramic element 10 is polarized in the thickness direction P as shown by an arrow in FIG. 1 by applying a high electric voltage thereacross. The piezoelectric ceramic element 10 is to generate and receive an ultrasonic oscillation. For this sake, a pair of electrode layers 14 and 15 are formed on the opposite surfaces of the piezoelectric ceramic element 10.
The ultrasonic probe shown in FIG. 1 further includes an acoustic matching section for acousticly matching the probe with the acoustic load (the object to be inspected), whereby making the probe to have a broad bandwidth and a low loss. The acoustic matching section, which is formed for broad-band matching with high efficiency between the piezoelectric ceramic element with relatively high acoustic impedance and a relatively low impedance acoustic load, is composed of two quarter-wave layers 11 and 12. The thickness of each quarter-wave layer 11 or 12 should be one quarter of the wavelength at the resonant frequency of the piezoelectric ceramic element 10.
The ultrasonic probe further includes a backing layer 13 for supporting the piezoelectric ceramic element 10 and absorbing the acoustic oscillation which propagates to the rear portion of the probe. The backing layer 13 should be designed to absorb as little power as possible and to maintain the passband characteristics.
In order to obtain excellent pulse-propagation characteristics with a broad bandwidth, low loss and low ripple, the quarter-wave layers 11 and 12 present respectively an acoustic impedance density (which is defined by the product of density and sonic velocity) of 8.0.times.10.sup.6 to 10.0.times.10.sup.6 Kg/m.sup.2.sec. and 2.0.times.10.sup.6 to 3.0.times.10.sup.6 Kg/m.sup.2.sec. For this sake, the quarter-wave layer 11 is conventionally made of a material such as silicate glass, chalcogenide glass and epoxy resin mixed with glass powder, and the quarter-wave layer 12 is made of a material such as epoxy resin and acrylic resin and the like.
In the case of a quarter-wave layer 11 of a glass plate, the glass plate is finely ground to present parallel and flat opposite surfaces and bonded by adhesive to the electrode layer 15. In the case of a quarter-wave layer of a resin or a resin mixed with an appropriate amount of fine glass powder, the resin is formed to a sheet and then fixed by adhesive to the electrode layer 15 or the quarter-wave layer 11. Alternatively, the quarter-wave layer 12 of resin is directly casted thereon.
When a high-frequency ultrasonic probe is prepared by such a conventional process, however, the quarter-wave layer 11 and 12 should be made with a smaller thickness which is in inverse proportion to the raised central frequency of the probe. In such a case, there has been a problem that the provision of the adhesive layer between the electrode layer 15 and the quarter-wave layer 11 and the quarter-wave layers 11 and 12 adversely affects the acoustic characteristics of the resulting ultrasonic probe.
Further, in the case of a quarter-wave layer of an organic resin mixed with glass powder or a quarter-wave layer made of an organic resin itself, it is difficult to obtain a resin sheet with a uniform thickness so that ultrasonic probes can hardly be prepared with a constant high performance. Further such a resin sheet tends to readily involve pinholes, resulting in a decrease in the production yield thereof.
Accordingly, with the conventional process, it has been extremely difficult to prepare an ultrasonic probe having quarter-wave layers of a thickness lower than 100 microns with a high and constant production efficiency. Thus, most of the high-frequency ultrasonic probes of the prior art which have been practically used are the single quarter-wave matched type or at most double quarter-wave matched type. It has been impossible to prepare a high-frequency ultrasonic probe with three quarter-wave layers which would exhibit a broader bandwidth.